Acoustic band gap materials, so-called phononic crystals, are introduced as a new platform for sensing material properties in small cavities. The sensor employs specific transmission windows within the band gap to determine properties of one component that builds the phononic crystal. The dependence of the frequency where transmission takes place is correlated to material properties, specifically to the sound velocity of a liquid. This value is related to several parameters of practical interest like the concentration of one component in a mixture or conversion rate in a microreactor. The capability of the concept will be demonstrated with a one-dimensional arrangement of solid plates and liquid-filled cavities and a two-dimensional periodic arrangement of liquid-filled holes in a solid matrix. The properties of 1D phononic crystals will be analysed in terms of the effective acoustic impedance and the resulting transmission behaviour and experimentally verified. The transmission properties of the 2D phononic crystal will be modelled with the layer multiple-scattering theory. Similar features which can be employed for sensing purposes will be discussed.